Structure and method for metal gate stack oxygen concentration control using an oxygen diffusion barrier layer and a sacrificial oxygen gettering layer

ABSTRACT

A process is disclosed of forming metal replacement gates for NMOS and PMOS transistors with oxygen in the PMOS metal gates and metal atom enrichment in the NMOS gates such that the PMOS gates have effective work functions above 4.85 eV and the NMOS gates have effective work functions below 4.25 eV. Metal work function layers in both the NMOS and PMOS gates are oxidized to increase their effective work functions to the desired PMOS range. An oxygen diffusion blocking layer is formed over the PMOS gate and an oxygen getter is formed over the NMOS gates. A getter anneal extracts the oxygen from the NMOS work function layers and adds metal atom enrichment to the NMOS work function layers, reducing their effective work functions to the desired NMOS range. Processes and materials for the metal work function layers, the oxidation process and oxygen gettering are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Nonprovisional patentapplication Ser. No. 12/275,812, filed Nov. 21, 2008, the contents ofwhich are herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to the field of integrated circuits. Moreparticularly, this invention relates to replacement metal gates of MOStransistors in integrated circuits.

BACKGROUND OF THE INVENTION

N-channel metal oxide semiconductor (NMOS) and p-channel metal oxidesemiconductor (PMOS) transistors in advanced integrated circuitsfrequently have replacement metal gates to improve on-state currentdensities. Attaining desired effective work functions of the metal gatesin both NMOS and PMOS transistors without significantly increasingfabrication cost and complexity has been problematic.

SUMMARY OF THE INVENTION

The instant invention provides a process for forming an integratedcircuit which includes NMOS and PMOS transistors with metal replacementgates. The process forms gate work function metal layers in the PMOStransistors with effective work functions above 4.85 eV and gate workfunction metal layers in the NMOS transistors with effective workfunctions below 4.25 eV.

Work function metal layers with effective work functions approximatelyat mid-gap are formed in both the NMOS and PMOS gates. The work functionmetal layers are oxidized to increase their effective work functions tothe desired PMOS range. Various oxidation processes suitable for thisstep are disclosed.

An oxygen diffusion blocking layer is formed over the PMOS gate and anoxygen getter is formed over both gates. Materials for the oxygendiffusion blocking layer and the getter are disclosed. A getter annealextracts the oxygen from the NMOS work function layers into the gettermaterial and adds metal atoms from the getter into the NMOS workfunction layers, reducing their effective work functions to the desiredNMOS range.

The getter is removed and a metal replacement gate material is formed inboth NMOS and PMOS gates.

DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1A through FIG. 1M are cross-sections of an integrated circuitcontaining an NMOS transistor and a PMOS transistor formed according tothe instant invention, depicted in successive stages of fabrication.

DETAILED DESCRIPTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the invention.Several aspects of the invention are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide a full understanding of the invention. One skilled in therelevant art, however, will readily recognize that the invention can bepracticed without one or more of the specific details or with othermethods. In other instances, well-known structures or operations are notshown in detail to avoid obscuring the invention. The present inventionis not limited by the illustrated ordering of acts or events, as someacts may occur in different orders and/or concurrently with other actsor events. Furthermore, not all illustrated acts or events are requiredto implement a methodology in accordance with the present invention.

For the purposes of this disclosure, the term “eV” is understood to meanthe unit of energy known as electron-volt. One eV is substantially equalto 1.602×10⁻¹⁹ joules. The term “meV” is understood to meanmilli-electron volts. One meV is substantially equal to 1.602×10⁻²²joules.

For the purposes of this disclosure, the term “MOS transistor” will beunderstood to mean metal oxide semiconductor transistor.

For the purposes of this disclosure, the term “work function” will beunderstood to mean a potential energy difference, measured in eV,between an electron at the Fermi level in a material, to which the workfunction is being referenced, and an electron in a vacuum adjacent tothe material. For example, intrinsic silicon has a work function ofapproximately 4.6 eV at room temperature. N-type silicon with a dopingdensity between 3×10¹⁷ cm⁻³ and 3×10¹⁸ cm⁻³ has a work function ofapproximately 4.1 eV at room temperature. P-type silicon with a dopingdensity between 3×10¹⁷ cm⁻³ and 3×10¹⁸ cm⁻³ has a work function ofapproximately 5.1 eV at room temperature. The term “effective workfunction” will be understood to mean a calculated potential energy of anelectron in a gate in an MOS transistor which is consistent with anobserved flat-band potential and all substrate and gate interface chargeeffects.

For the purposes of this disclosure, the term “getter” will beunderstood to refer to a material which has a capability of absorbing anelement such as oxygen from a region surrounding the getter material.Similarly, the term “gettering” will be understood to mean the act ofabsorbing an element into a getter material.

Chemical formulas such as SiON of a material in this disclosure areunderstood to list elements of which the material is substantiallycomposed, but no stoichiometric relationships between the elements areimplied or may be assumed, unless numerical subscripts are included inthe chemical formulas.

The instant invention provides a process for forming an integratedcircuit which includes NMOS and PMOS transistors with metal replacementgates. The process forms gate work function metal layers in the PMOStransistors with effective work functions above 4.85 eV and gate workfunction metal layers in the NMOS transistors with effective workfunctions below 4.25 eV.

FIG. 1A through FIG. 1M are cross-sections of an integrated circuitcontaining an NMOS transistor and a PMOS transistor formed according tothe instant invention, depicted in successive stages of fabrication.Referring to FIG. 1A, the integrated circuit (1000) is formed on asemiconductor substrate (1002) which is commonly a single crystalsilicon wafer, but may be a silicon-on-insulator (SOI) wafer, a hybridorientation technology (HOT) wafer with regions of different crystalorientations, or other material appropriate for fabrication of the IC(1000). The integrated circuit (1000) contains an NMOS region (1004) foran NMOS transistor and a PMOS region (1006) for a PMOS transistor. AnNMOS gate dielectric layer (1008) is formed on a top surface of thesubstrate (1002) in the NMOS region (1004). The NMOS gate dielectriclayer (1008) is typically one or more layers of silicon oxide (SiO_(x)),silicon nitride (Si₃N₄), silicon oxy-nitride (SiON), aluminum oxide(Al₂O₃), aluminum oxy-nitride (AlON), hafnium oxide (HfO), hafniumsilicate (HfSiO), hafnium silicon oxy-nitride (HfSiON), zirconium oxide(ZrO), zirconium silicate (ZrSiO), zirconium silicon oxy-nitride(ZrSiON), a combination of the aforementioned materials, or otherinsulating material. The NMOS gate dielectric layer (1008) may includenitrogen as a result of exposure to a nitrogen containing plasma or anitrogen containing ambient gas at temperatures between 50° C. and 800°C. The NMOS gate dielectric layer (1008) is typically between 1 and 4nanometers thick. A thicker NMOS gate dielectric layer (1008) may beformed in NMOS transistors operating above 2.5 volts. The NMOS gatedielectric layer (1008) may be formed by any of a variety of gatedielectric formation processes, for example thermal oxidation, plasmanitridation of an oxide layer, and/or dielectric material deposition byatomic layer deposition (ALD).

A PMOS gate dielectric layer (1010) is formed on a top surface of thesubstrate (1002) in the PMOS region (1006). The PMOS gate dielectriclayer (1010) is typically between 1 and 4 nanometers thick, and includessome or all of the same materials in the NMOS gate dielectric layer(1008). A thicker PMOS gate dielectric layer (1010) may be formed inPMOS transistors operating above 2.5 volts. All or part of the PMOS gatedielectric layer (1010) is typically formed concurrently with the NMOSgate dielectric layer (1008).

An NMOS gate work function metal layer (1012) is formed on a top surfaceof the NMOS gate dielectric layer (1008). The NMOS gate work functionmetal layer (1012) may be titanium nitride (TiN), tantalum nitride(TaN), tantalum carbide (TaC), or other metal with an effective workfunction between 4.5 and 4.7 eV. The NMOS gate work function metal layer(1012) may be between 1 and 10 nanometers thick, and may be formed byphysical vapor deposition (PVD), chemical vapor deposition (CVD),metal-organic chemical vapor deposition (MOCVD), ALD, or other processappropriate for deposition of thin metal films. A PMOS gate workfunction metal layer (1014) is formed on a top surface of the PMOS gatedielectric layer (1010) of similar materials and properties as the NMOSgate work function metal layer (1012). In one embodiment, the PMOS gatework function metal layer (1014) may be formed concurrently with theNMOS gate work function metal layer (1012).

An NMOS dummy gate layer (1016) is formed on a top surface of the NMOSgate work function metal layer (1012). The NMOS dummy gate layer (1016)may be polycrystalline silicon, commonly known as polysilicon, or othermaterial such as silicon germanium which has a high etch selectivity tothe NMOS gate work function metal layer (1012) during a subsequent wetetch process. In one embodiment, the NMOS dummy gate layer (1016) isbetween 40 and 80 nanometers thick. In another embodiment, the thicknessof the NMOS dummy gate layer (1016) is between two and three times awidth of an NMOS gate to be formed in the NMOS region (1004). The NMOSdummy gate layer (1016) may be formed by plasma enhanced chemical vapordeposition (PECVD) or other deposition process appropriate for formingthin films of dummy gate material. In an alternate embodiment, the NMOSdummy gate layer (1016) may be doped with phosphorus to improve etchselectivity to the NMOS gate work function metal layer (1012) during thesubsequent wet etch process.

A PMOS dummy gate layer (1018) is formed on a top surface of the PMOSgate work function metal layer (1014) of similar materials andproperties as the NMOS dummy gate layer (1016). In one embodiment, thePMOS dummy gate layer (1018) may be formed concurrently with the NMOSdummy gate layer (1016).

An NMOS hard mask (1020) is formed on a top surface of the NMOS dummygate layer (1016). The NMOS hard mask (1020) may include one or morelayers of silicon nitride (SiN), silicon oxy-nitride (SiON), siliconcarbide (SiC), silicon oxy-carbide (SiOC), silicon oxy-nitride-carbide(SiCON), or other dielectric material appropriate for blockingsilicidation of the top surface of the NMOS dummy gate layer (1016)during a subsequent silicidation process, and appropriate for providinga stop layer for a subsequent chemical mechanical polish (CMP) process.The NMOS hard mask (1020) may be formed by PVD, CVD, PECVD, MOCVD orother deposition process. In one embodiment, the NMOS hard mask (1020)may be between 20 and 40 nanometers thick.

A PMOS hard mask (1022) is formed on a top surface of the PMOS dummygate layer (1018) of similar materials and properties as the NMOS hardmask (1020). In one embodiment, the PMOS hard mask (1022) may be formedconcurrently with the NMOS hard mask (1020).

Referring to FIG. 1B, unwanted material from the NMOS hard mask (1020),the NMOS dummy gate layer (1016), the NMOS gate work function metallayer (1012) and the NMOS gate dielectric layer (1008) is removed duringa gate etch process. Similarly, unwanted material from the PMOS hardmask (1022), the PMOS dummy gate layer (1018), the PMOS gate workfunction metal layer (1014) and the PMOS gate dielectric layer (1010) isremoved during the gate etch process. A gate photoresist pattern, notshown in FIG. 1B for clarity, is formed on top surfaces of the NMOS hardmask (1020) and the PMOS hard mask (1022). The gate etch processincludes reactive ion etch (RIE) process steps which provide fluorinecontaining etchants and possibly chlorine containing etchants from aplasma to a top surface of the integrated circuit (1000). Thephotoresist pattern blocks the etchants from areas of the NMOS region(1004) and PMOS region (1006) defined for an NMOS gate and a PMOS gate.

NMOS gate sidewall spacers (1024) are formed on lateral surfaces of theNMOS dummy gate layer (1016), the NMOS gate work function metal layer(1012) and the NMOS gate dielectric layer (1008), typically bydeposition of one or more conformal layers of silicon nitride and/orsilicon oxide on a top and lateral surfaces of the NMOS gate layers andthe top surface of the substrate (1002), followed by removal of theconformal layer material from the top surface of the NMOS hard mask(1020) and the top surface of the substrate (1002) by anisotropicetching methods, leaving the conformal layer material on the lateralsurfaces of the NMOS dummy gate layer (1016), the NMOS gate workfunction metal layer (1012) and the NMOS gate dielectric layer (1008).The NMOS gate sidewall spacers (1024) are typically 10 to 50 nanometersthick. Similarly, PMOS gate sidewall spacers (1026) are formed onlateral surfaces of the PMOS dummy gate layer (1018), the PMOS gate workfunction metal layer (1014) and the PMOS gate dielectric layer (1010).It is typical for a part of the PMOS gate sidewall spacers (1026) to beformed concurrently with the NMOS gate sidewall spacers (1024).

N-type source and drain (NSD) regions (1028), including n-type lightlydoped drain (NLDD) regions, are formed in the substrate (1002) adjacentto the NMOS gate dielectric layer (1008) by ion implanting n-typedopants such as phosphorus and arsenic into the substrate (1002). NLDDregions are formed by implanting n-type NLDD dopants adjacent to theNMOS gate dielectric layer (1008) prior to formation of the NMOS gatesidewall spacers (1024). Additional n-type NSD dopants are implantedafter formation of the NMOS gate sidewall spacers (1024) and annealed toform the NSD regions (1028). Similarly, p-type source and drain (PSD)regions (1030), including p-type lightly doped drain (PLDD) regions, areformed in the substrate (1002) adjacent to the PMOS gate dielectriclayer (1010) by ion implanting p-type dopants such as boron and possiblygallium into the substrate (1002). PLDD regions are formed by implantingp-type PLDD dopants adjacent to the PMOS gate dielectric layer (1010)prior to formation of the PMOS gate sidewall spacers (1026). Additionalp-type PSD dopants are implanted after formation of the PMOS gatesidewall spacers (1026) and annealed to form the PSD regions (1030).

NSD metal silicide layers (1032) are formed on top surfaces of the NSDregions (1028) and PSD metal silicide layers (1034) are formed on topsurfaces of the PSD regions (1030). The metal silicide layers (1032,1034) are formed on exposed silicon areas of the top surface ofintegrated circuit (1000), commonly by depositing a layer of metal, suchas nickel, cobalt, or titanium, on a top surface of the IC (1000),heating the IC (1000) to react a portion of the metal with exposedsilicon in active areas of the IC (1000), and selectively removingunreacted metal from the IC (1000) surface, commonly by exposing the IC(1000) to wet etchants including a mixture of an acid and hydrogenperoxide. The NMOS hard mask (1020) desirably blocks formation of metalsilicide on the top surface of the NMOS dummy gate layer (1016), and thePMOS hard mask (1022) desirably blocks formation of metal silicide onthe top surface of the PMOS dummy gate layer (1018). It is desirable toblock formation of metal silicide on the top surfaces of the NMOS dummygate layer (1016) and the PMOS dummy gate layer (1018) so as to improveetch characteristics of the dummy gate layers (1016, 1020) during asubsequent dummy gate removal process.

Referring to FIG. 1C, a conformal fill oxide layer (1036) is formed onan existing top surface of the integrated circuit (1000). In oneembodiment, the fill oxide layer (1036) is composed substantially ofsilicon dioxide, formed by thermal decomposition of tetraethylorthosilicate, also known as tetraethoxysilane or TEOS, by thermalcuring of methylsilsesquioxane (MSQ), by a high density plasma (HDP)process, by an ozone based thermal chemical vapor deposition (CVD)process, also known as a high aspect ratio process (HARP), by a lowpressure chemical vapor deposition (LPCVD) process or by an atmosphericpressure chemical vapor deposition (APCVD) process. The fill oxide layer(1036) is removed from top surfaces of the NMOS hard mask (1020) and thePMOS hard mask (1022) by a selective removal process such as an oxideCMP process. The NMOS hard mask (1020) and the PMOS hard mask (1022)desirably provide stop layers for the CMP process.

FIG. 1D depicts the integrated circuit (1000) after an etchback process(1038) which removes the NMOS hard mask (1020) and the PMOS hard mask(1022), and removes material from top surfaces of the NMOS gate sidewallspacers (1024), the PMOS gate sidewall spacers (1026) and the fill oxidelayer (1036). In one embodiment, the etchback process (1038) providessubstantially equal etch rates of the NMOS hard mask (1020), the PMOShard mask (1022), the NMOS gate sidewall spacers (1024), the PMOS gatesidewall spacers (1026) and the fill oxide layer (1036). The etchbackprocess (1038) may include an RIE step using fluorine containingetchants and possibly oxygen radicals and/or ions.

FIG. 1E depicts the integrated circuit (1000) after a dummy gate removalprocess. In one embodiment of the dummy gate removal process, theintegrated circuit (1000) is exposed to wet etchants which includeaqueous ammonium hydroxide (NH₄OH) at a concentration between 0.5 and 5percent, at a temperature between 25° C. and 50° C., for 20 to 150seconds. In another embodiment of the dummy gate removal process, theintegrated circuit (1000) is exposed to wet etchants which include anaqueous mixture of NH₄OH and tetra-methyl ammonium hydroxide (TMAH). Ina further embodiment, the integrated circuit (1000) is exposed to wetetchants which include choline. The NMOS dummy gate layer material andthe PMOS dummy gate layer material is removed from the NMOS region(1004) and the PMOS region (1006) while providing an etch selectivity tothe NMOS gate work function metal layer (1012) and the PMOS gate workfunction metal layer (1014) of more than 100:1.

FIG. 1F depicts the integrated circuit (1000) during a low temperatureoxidation process. Oxygen atoms (1040) are provided by the lowtemperature oxidation process to an existing top surface of theintegrated circuit (1000), and diffuse in to the NMOS gate work functionmetal layer (1012) and the PMOS gate work function metal layer (1014).In one embodiment, the oxygen atoms (1040) in the gate work functionmetal layers (1012, 1014) may have a distribution of at least 1×10¹⁵atoms/cm² within 1 nanometer of the top surfaces of the NMOS gatedielectric layer (1008) and the PMOS gate dielectric layer (1010). Inanother embodiment, the oxygen atoms (1040) in the gate work functionmetal layers (1012, 1014) may have an average concentration between1×10¹⁸ atoms/cm³ and 1×10²¹ atoms/cm³. The oxygen atoms (1040) in thegate work function metal layers (1012, 1014) increase the effective workfunctions of the gate work function metal layers (1012, 1014) to above4.85 eV.

In a first embodiment of the low temperature oxidation process, theintegrated circuit (1000) may be exposed to a steam ambient between 300°C. and 600° C. for 10 seconds to 30 minutes, as described in U.S. PatentProvisional Application ______ (Texas Instruments docket numberTI-67192, filed mm/dd/yyyy) hereby incorporated by reference. In analternate embodiment, the integrated circuit may be exposed to a steamambient between 900° C. and 1100° C. for less than 1 second, asdescribed in U.S. Patent Provisional Application ______ (TexasInstruments docket number TI-67192, filed mm/dd/yyyy).

In a second embodiment of the low temperature oxidation process, theintegrated circuit (1000) may be exposed to a plasma containing oxygenand hydrogen at a temperature up to 500° C., as described in U.S. PatentProvisional Application ______ (Texas Instruments docket numberTI-67192, filed mm/dd/yyyy). In an alternate embodiment, the integratedcircuit may be exposed to a plasma containing oxygen, and subsequentlyexposed to a plasma containing hydrogen, as described in U.S. PatentProvisional Application ______ (Texas Instruments docket numberTI-67192, filed mm/dd/yyyy).

In a third embodiment of the low temperature oxidation process, the gatework function metal layers (1012, 1014) may be anodized at a temperatureup to 100° C., as described in U.S. Patent Provisional Application______ (Texas Instruments docket number TI-67192, filed mm/dd/yyyy).

In a fourth embodiment of the low temperature oxidation process, theintegrated circuit (1000) may be exposed to a dry ambient containing anoxidizing component, such as oxygen (O₂), ozone (O₃), nitric oxide (NO),nitrous oxide (NO₂), or any combination of these gases. The ambient mayalso include inert gases such as nitrogen or argon. The integratedcircuit (1000) may be heated to 300° C. to 700° C. during exposure tothe dry ambient.

In a fifth embodiment of the low temperature oxidation process, theintegrated circuit (1000) may be exposed to H₂O containing dissolvedozone.

FIG. 1G depicts the integrated circuit (1000) after formation of an etchstop layer and an oxygen diffusion barrier layer. A thin etch stop layer(1042) is formed on an existing top surface of the integrated circuit(1000) including the top surfaces of the NMOS gate work function metallayer (1012) and the PMOS gate work function metal layer (1014). Theetch stop layer (1042) is between 2 and 10 nanometers thick over theNMOS gate work function metal layer (1012) and the PMOS gate workfunction metal layer (1014), and is formed of a metallic material whichallows oxygen to diffuse across the etch stop layer (1042) during asubsequent getter anneal process step, and provides an etch stopcapability during a subsequent oxygen diffusion barrier etch processstep, such as TaN, tungsten nitride (WN), titanium carbide (TiC), TaC,or tungsten carbide (WC). The etch stop layer (1042) may be formed byALD, CVD, PVD, MOCVD or other suitable deposition method. An oxygendiffusion barrier layer (1044) is formed on a top surface of the etchstop layer (1042). The oxygen diffusion barrier layer (1044) is greaterthan 10 nanometers thick over the NMOS gate work function metal layer(1012) and the PMOS gate work function metal layer (1014), and is formedof a metallic material which blocks oxygen diffusion during thesubsequent getter anneal process step, and has a high etch rateselectivity to the etch stop layer (1042) during the subsequent oxygendiffusion barrier etch process step, such as TiN. The oxygen diffusionbarrier layer (1044) may be formed by ALD, CVD, PVD, MOCVD or othersuitable deposition method.

FIG. 1H depicts the integrated circuit (1000) during a process step toremove the oxygen diffusion barrier layer (1044) in the NMOS region(1004). An oxygen diffusion barrier photoresist pattern (1046) is formedon a top surface of the oxygen diffusion barrier layer (1044) in thePMOS region (1006). An oxygen diffusion barrier etch process (1048) isperformed which removes substantially all the oxygen diffusion barrierlayer (1044) in the NMOS region (1004) from the top surface of the etchstop layer (1042) in direct contact with the NMOS gate work functionmetal layer (1012). The oxygen diffusion barrier etch process (1048) maybe an RIE process using chlorine etchants and possibly fluorineetchants. After the oxygen diffusion barrier etch process (1048) iscompleted, the oxygen diffusion barrier photoresist pattern (1046) isremoved from the top surface of the oxygen diffusion barrier layer(1044), for example by exposing the integrated circuit (1000) to anoxygen containing plasma, followed by a wet cleanup to remove anyorganic residue from the top surface of the oxygen diffusion barrierlayer (1044).

FIG. 1I depicts the integrated circuit (1000) after formation of aoxygen getter layer (1050) on the top surface of the etch stop layer(1042) over the NMOS gate work function metal layer (1012) and on thetop surface of the oxygen diffusion barrier layer (1044) over the PMOSgate work function metal layer (1014). In one embodiment, the oxygengetter layer (1050) may be formed on an existing top surface of theintegrated circuit (1000). The oxygen getter layer (1050) may includetitanium, hafnium, zirconium, tantalum, aluminum, cerium, lanthanum, orother metal capable of gettering oxygen atoms from the NMOS gate workfunction metal layer (1012) during a subsequent getter anneal processstep. The oxygen getter layer (1050) may be formed by ALD, CVD, PVD,MOCVD or other suitable deposition method.

FIG. 1J depicts the integrated circuit (1000) during the getter annealprocess step, in which the integrated circuit (1000) is heated to causethe oxygen atoms (1040) to diffuse from the NMOS gate work functionmetal layer (1012) through the etch stop layer (1042) into the oxygengetter layer (1050) in the NMOS region (1004). During the getter annealprocess, metal atoms (1052) diffuse from the oxygen getter layer (1050)in the NMOS region (1004) through the etch stop layer (1042) into NMOSgate work function metal layer (1012). The effect of the getter annealprocess is to reduce the effective work function of the NMOS gate workfunction metal layer (1012) to a value below 4.25 eV. After the getteranneal process is completed, the oxygen atoms (1040) in the NMOS gatework function metal layer (1012) have a distribution of less than 1×10¹³atoms/cm² within 1 nanometer of the top surface of the NMOS gatedielectric layer (1008). Metal atoms (1052) from the oxygen getter layer(1050) have a distribution of at least 1×10¹³ atoms/cm² within 1nanometer of the top surface of the NMOS gate dielectric layer (1008).

Oxygen gettering in the PMOS region (1006) is blocked by the oxygendiffusion barrier layer (1044). Less than 10 percent of the oxygen atoms(1040) in the PMOS gate work function metal layer (1014) are removedduring the getter anneal process, so that the effective work function ofthe PMOS gate work function metal layer (1014) remains above 4.85 eV.

The getter anneal process is performed at less than 600° C. to avoidunwanted reactions in the silicide layers (1032, 1034). In oneembodiment, the getter anneal process is performed in an inert ambientsuch as nitrogen or argon. In an alternate embodiment, the getter annealprocess is performed at an ambient pressure below 1 millitorr.

FIG. 1K depicts the integrated circuit (1000) after removing the oxygengetter layer. An oxygen getter etch process is performed, which mayinclude immersion in an aqueous solution of a mixture of hydrogenperoxide and an alkaline etchant such as NH₄OH. All or part of theoxygen diffusion barrier layer (1044) may be removed from the PMOSregion (1006). All or part of the etch stop layer (1042) in directcontact with the NMOS gate work function metal layer (1012) may beremoved during the oxygen getter etch process. Similarly, all or part ofthe etch stop layer (1042) in direct contact with the PMOS gate workfunction metal layer (1014) may be removed during the oxygen getter etchprocess.

FIG. 1L depicts the integrated circuit (1000) after formation of a metalfill gate layer (1054) over the NMOS gate work function metal layer(1012) and the PMOS gate work function metal layer (1014). In oneembodiment, the metal fill gate layer (1054) may be formed on anexisting top surface of the integrated circuit (1000). The metal fillgate layer (1054) may be aluminum, tungsten, or any metal with asuitably low electrical resistivity. The metal fill gate layer (1054)may be formed by ALD, CVD, PVD, MOCVD or other suitable depositionmethod.

FIG. 1M depicts the integrated circuit (1000) after a planarizingprocess which removes the metal fill gate layer, the oxygen diffusionbarrier layer (1044) if present, and the etch stop layer (1042) ifpresent, from the top surface of the fill oxide layer (1036). Materialfrom the metal fill gate layer is left over the NMOS gate work functionmetal layer (1012) to form an NMOS metal fill gate (1056) and is leftover the PMOS gate work function metal layer (1014)) to form a PMOSmetal fill gate (1058). The planarizing process may be a CMP process, anisotropic chemical or physical etch process, or a combinations thereof.

Formation of the integrated circuit (1000) is continued using knownprocesses.

What is claimed is:
 1. A process of forming an MOS transistor,comprising the steps of: forming a dummy gate stack, wherein said dummygate stack includes a gate dielectric layer, a gate work function metallayer and a dummy gate layer; removing said dummy gate layer; performinga low temperature oxidation process, such that said effective workfunction of said gate work function metal layer is increased to a valueabove 4.85 eV; forming an oxygen diffusion barrier layer over said gatework function metal layer; forming an oxygen getter layer over said gatework function metal layer; performing a getter anneal process; andforming a metal gate over said gate work function metal layer.
 2. Theprocess of claim 1, in which said gate work function metal layer has aneffective work function between 4.5 and 4.7 eV.
 3. The process of claim1, in which said step of removing said dummy gate layer further includesthe steps of: forming a hard mask on a top surface of said dummy gatelayer; forming a gate photoresist pattern on a top surface of said hardmask, such that said gate photoresist pattern defines an area for a gateof said MOS transistor; performing a gate etch process, said gate etchprocess further including the steps of: removing said hard mask outsideof said gate area of said MOS transistor; removing said dummy gate layeroutside of said gate area of said MOS transistor; removing said gatework function metal layer outside of said gate area of said MOStransistor; and removing said gate dielectric layer outside of said gatearea of said MOS transistor; forming a fill oxide layer on a top surfaceof said substrate and a top surface of said hard mask; removing saidfill oxide layer from said top surface of said hard mask in said gatearea of said MOS transistor; removing said hard mask in said gate areaof said MOS transistor; and removing said dummy gate layer in said gatearea of said MOS transistor.
 4. The process of claim 3, wherein: saidhard mask is between 20 and 40 nanometers thick; and a composition ofsaid hard mask is selected from the group consisting of: SiN, SiON, SiC,SiOC, SiCON, and any combination thereof.
 5. The process of claim 3, inwhich said step of forming an oxygen diffusion barrier layer over saidgate work function metal layer further includes the step of forming anetch stop layer on a top surface of said gate work function metal layerprior to forming said oxygen diffusion barrier layer.
 6. The process ofclaim 5, wherein: said etch stop layer is between 2 and 10 nanometersthick; and a composition of said etch stop layer is selected from thegroup consisting of: TaN, WN, TiC, TaC, and WC.
 7. The process of claim6, in which said step of forming a metal gate further includes the stepsof: removing said oxygen getter layer; forming a metal fill gate layerover said gate work function metal layer and on a top surface of saidfill oxide layer; and removing said metal fill gate layer from said topsurface of said fill oxide layer.
 8. The process of claim 7, wherein:said MOS transistor is a PMOS transistor; said gate work function metallayer includes oxygen atoms such that said oxygen atoms have adistribution of at least 1×10¹⁵ atoms/cm² within 1 nanometer of said topsurface of said gate dielectric layer; and said gate work function metallayer has an effective work function above 4.85 eV.
 9. The process ofclaim 7, wherein: said MOS transistor is a PMOS transistor; said gatework function metal layer includes oxygen atoms such that said oxygenatoms have an average concentration between 1×10¹⁸ atoms/cm³ and 1×10²¹atoms/cm³; and said gate work function metal layer has an effective workfunction above 4.85 eV.
 10. The process of claim 7, wherein: said MOStransistor is an NMOS transistor; said process of forming saidtransistor further includes the step of removing said oxygen diffusionbarrier layer, such that said step of removing said oxygen diffusionbarrier layer is performed after said step of forming an oxygendiffusion barrier layer and before said step of forming an oxygen getterlayer; said gate work function metal layer includes oxygen atoms suchthat said oxygen atoms have a distribution less than 1×10¹³ oxygenatoms/cm² within 1 nanometer of said top surface of said gate dielectriclayer; said gate work function metal layer includes metal atoms fromsaid oxygen getter layer such that said metal atoms have a distributionof at least 1×10¹³ atoms/cm² within 1 nanometer of said top surface ofsaid gate dielectric layer; and said gate work function metal layer hasan effective work function below 4.25 eV.
 11. The process of claim 1,wherein a composition of said gate dielectric layer is selected from thegroup consisting of: SiO_(x), Si₃N₄, SiON, Al₂O₃, AlON, HfO, HfSiO,HfSiON, ZrO, ZrSiO, ZrSiON, nitridated SiO_(x), nitridated Al₂O₃,nitridated HfO, nitridated HfSiO, nitridated ZrO, nitridated ZrSiO, andany combination thereof.
 12. The process of claim 1, wherein: said gatework function metal layer is between 1 and 10 nanometers thick; and saidgate work function metal layer includes a metal selected from the groupconsisting of: TiN, TaN, and TaC.
 13. The process of claim 1, whereinsaid step of performing a low temperature oxidation process furtherincludes the step of exposing said gate work function metal layer to asteam ambient between 300° C. and 600° C. for 10 seconds to 30 minutes.14. The process of claim 1, wherein said step of performing a lowtemperature oxidation process further includes the step of exposing saidgate work function metal layer to a plasma containing oxygen andhydrogen at a temperature up to 500° C.
 15. The process of claim 1,wherein said step of performing a low temperature oxidation processfurther includes the step of anodizing said gate work function metallayer.
 16. The process of claim 1, wherein said step of performing a lowtemperature oxidation process further includes the step of exposing saidgate work function metal layer to dry ambient containing an oxidizingcomponent at a temperature between 300° C. and 700° C.
 17. The processof claim 16, wherein said oxidizing component is selected from the groupconsisting of: O₂, O₃, NO, NO₂, and any combination thereof.
 18. Theprocess of claim 1, wherein said step of performing a low temperatureoxidation process further includes the step of exposing said gate workfunction metal layer to H₂O containing dissolved ozone.
 19. The processof claim 1, wherein said oxygen diffusion barrier layer is more than 10nanometers thick.
 20. The process of claim 1, wherein a composition ofsaid oxygen getter layer is selected from the group consisting of:titanium, hafnium, zirconium, tantalum, aluminum, cerium, and lanthanum.21. A process of forming an integrated circuit, comprising the steps of:forming a dummy PMOS gate stack and a dummy NMOS gate stack, whereinsaid dummy PMOS gate stack includes a PMOS gate dielectric layer, a PMOSgate work function metal layer and a PMOS dummy gate layer, and saiddummy NMOS gate stack includes a NMOS gate dielectric layer, a NMOS gatework function metal layer and a NMOS dummy gate layer; removing saidPMOS dummy gate layer and said NMOS dummy gate layer; performing a lowtemperature oxidation process, such that said effective work function ofsaid NMOS gate work function metal layer is increased to a value above4.85 eV and said effective work function of said PMOS gate work functionmetal layer is increased to a value above 4.85 eV; forming an oxygendiffusion barrier layer over said PMOS gate work function metal layer;forming an oxygen getter layer over said NMOS gate work function metallayer, wherein said oxygen getter layer is blocked from said PMOS gatework function metal layer by said oxygen diffusion barrier layer;performing a getter anneal process, such that said effective workfunction of said NMOS gate work function metal layer is reduced to avalue below 4.25 eV and said effective work function of said PMOS gatework function metal layer is maintained at a value above 4.85 eV; andforming an NMOS metal gate over said NMOS gate work function metal layerand a PMOS metal gate over said PMOS gate work function metal layer. 22.The process of claim 21, in which: said PMOS gate work function metallayer has an effective work function between 4.5 and 4.7 eV; and saidNMOS gate work function metal layer has an effective work functionbetween 4.5 and 4.7 eV.
 23. The process of claim 21, in which said stepof removing said PMOS dummy gate layer and said NMOS dummy gate layerfurther includes the steps of: forming a hard mask on a top surface ofsaid dummy gate layer; forming a gate photoresist pattern on a topsurface of said hard mask, such that said gate photoresist patterndefines an area for an NMOS gate of an NMOS transistor and defines anarea for a PMOS gate of a PMOS transistor; performing a gate etchprocess, said gate etch process further including the steps of: removingsaid hard mask outside of said areas for said NMOS gate and said PMOSgate; removing said dummy gate layer outside of said areas for said NMOSgate and said PMOS gate; removing said NMOS gate work function metallayer outside of said NMOS gate area; removing said PMOS gate workfunction metal layer outside of said PMOS gate area; removing said NMOSgate dielectric layer outside of said NMOS gate area; and removing saidPMOS gate dielectric layer outside of said PMOS gate area; forming afill oxide layer on a top surface of said substrate and a top surface ofsaid hard mask; removing said fill oxide layer from said top surface ofsaid hard mask in said areas for said NMOS gate and said PMOS gate;removing said hard mask in said areas for said NMOS gate and said PMOSgate; and removing said dummy gate layer in said areas for said NMOSgate and said PMOS gate.
 24. The process of claim 23, wherein: said hardmask is between 20 and 40 nanometers thick; and a composition of saidhard mask is selected from the group consisting of: SiN, SiON, SiC,SiOC, SiCON, and any combination thereof.
 25. The process of claim 23,in which said step of forming an oxygen diffusion barrier layer furtherincludes the step of forming an etch stop layer on said top surface ofsaid NMOS gate work function metal layer and on said top surface of saidNMOS gate work function metal layer prior to forming said oxygendiffusion barrier layer.
 26. The process of claim 25, wherein: said etchstop layer is between 2 and 10 nanometers thick; and a composition ofsaid etch stop layer is selected from the group consisting of: TaN, WN,TiC, TaC, and WC.
 27. The process of claim 26, in which said step offorming an oxygen diffusion barrier layer further includes the step offorming said oxygen diffusion barrier layer on a top surface of saidetch stop layer.
 28. The process of claim 27, in which said step offorming an oxygen getter layer over said NMOS gate work function metallayer further includes the steps of: forming an oxygen diffusion barrierphotoresist pattern on a top surface of said oxygen diffusion barrierlayer, such that said oxygen diffusion barrier layer is exposed in saidarea for said NMOS gate; removing said oxygen diffusion barrier layer insaid area for said NMOS gate; and forming said oxygen getter layer onsaid top surface of said etch stop layer in said area for said NMOS gateand on said top surface of oxygen diffusion barrier layer in said areafor said PMOS gate.
 29. The process of claim 28, in which said step offorming an NMOS metal gate and a PMOS metal gate further includes thesteps of: removing said oxygen getter layer; forming a metal fill gatelayer over said NMOS gate work function metal layer, over said PMOS gatework function metal layer and on a top surface of said fill oxide layer;and removing said metal fill gate layer from said top surface of saidfill oxide layer.
 30. The process of claim 21, wherein: a composition ofsaid NMOS gate dielectric layer and a composition of said PMOS gatedielectric layer are each selected from the group consisting of:SiO_(x), Si₃N₄, SiON, Al₂O₃, AlON, HfO, HfSiO, HfSiON, ZrO, ZrSiO,ZrSiON, nitridated SiO_(x), nitridated Al₂O₃, nitridated HfO, nitridatedHfSiO, nitridated ZrO, nitridated ZrSiO, and any combination thereof.31. The process of claim 21, wherein: said NMOS gate work function metallayer is between 1 and 10 nanometers thick; said PMOS gate work functionmetal layer is between 1 and 10 nanometers thick; and said NMOS gatework function metal layer and said PMOS gate work function metal layereach include a metal selected from the group consisting of: TiN, TaN,and TaC.
 32. The process of claim 21, wherein said step of performing alow temperature oxidation process further includes the step of exposingsaid NMOS gate work function metal layer and said PMOS gate workfunction metal layer to a steam ambient between 300° C. and 600° C. for10 seconds to 30 minutes.
 33. The process of claim 21, wherein said stepof performing a low temperature oxidation process further includes thestep of exposing said NMOS gate work function metal layer and said PMOSgate work function metal layer to a plasma containing oxygen andhydrogen at a temperature up to 500° C.
 34. The process of claim 21,wherein said step of performing a low temperature oxidation processfurther includes the step of anodizing said NMOS gate work functionmetal layer and said PMOS gate work function metal layer.
 35. Theprocess of claim 21, wherein said step of performing a low temperatureoxidation process further includes the step of exposing said NMOS gatework function metal layer and said PMOS gate work function metal layerto dry ambient containing an oxidizing component at a temperaturebetween 300° C. and 700° C.
 36. The process of claim 35, wherein saidoxidizing component is selected from the group consisting of: O₂, O₃,NO, NO₂, and any combination thereof.
 37. The process of claim 21,wherein said step of performing a low temperature oxidation processfurther includes the step of exposing said gate work function metallayer to H₂O containing dissolved ozone.
 38. The process of claim 21,wherein said oxygen diffusion barrier layer is more than 10 nanometersthick.
 39. The process of claim 21, wherein a composition of said oxygengetter layer is selected from the group consisting of: titanium,hafnium, zirconium, tantalum, aluminum, cerium, and lanthanum.
 40. Theprocess of claim 21, wherein said PMOS gate work function metal layerincludes oxygen atoms such that said oxygen atoms have a distribution ofat least 1×10¹⁵ atoms/cm² within 1 nanometer of said top surface of saidgate dielectric layer.
 41. The process of claim 21, wherein said PMOSgate work function metal layer includes oxygen atoms such that saidoxygen atoms have an average concentration between 1×10¹⁸ atoms/cm³ and1×10²¹ atoms/cm³.
 42. The process of claim 21, wherein: said NMOS gatework function metal layer includes oxygen atoms such that said oxygenatoms have a distribution less than 1×10¹³ oxygen atoms/cm² within 1nanometer of said top surface of said NMOS gate dielectric layer; andsaid NMOS gate work function metal layer includes metal atoms from saidoxygen getter layer such that said metal atoms have a distribution of atleast 1×10¹³ atoms/cm² within 1 nanometer of said top surface of saidNMOS gate dielectric layer.